The present invention relates to an apparatus for controlling the speed of a vehicle with an internal combustion engine, more particularly to a vehicle speed control apparatus by which the speed of a vehicle having an electronically controlled internal combustion engine is controlled so as to maintain a predetermined speed.
In prior art, a number of vehicle speed control apparatuses have been proposed in which the amount of fuel supplied to an internal combustion engine such as a diesel engine, gasoline engine or the like is regulated so as to control the speed of the vehicle having the engine at a desired constant speed. Such an apparatus is required to change the control condition in accordance with the condition of engine operation in order to carry out satisfactory vehicle speed control. For example, as disclosed in Japanese Utility Model Public Disclosure No. 57-14212, a vehicle speed control apparatus has been proposed in which a vehicle speed pulse with a frequency corresponding to the vehicle speed is converted into a voltage signal by a frequency-voltage converter, the conversion gain is changed in accordance with a predetermined level by the use of a changing circuit, whereby the gain of the control loop is changed in accordance with the vehicle speed.
However, since the apparatus gives no consideration to increases in engine load which occur, for example, during hill climbing and takes account of only the vehicle speed, it is difficult to attain satisfactory vehicle speed control. That is, in order to control the vehicle speed with good response characteristics, it is necessary to increase the gain of the control loop when the engine load increases. However, in the conventional apparatus, the control characteristics are insufficient since no consideration is given to this point.